The present invention relates to diesel fuel compositions containing certain azole compounds as cetane improvers.
Diesel engines operate by compression ignition. They have compression ratios in the range of 14:1 to 17:1 or higher and for that reason obtain more useful work from a given amount of fuel compared to an Otto cycle engine. Historically, diesel engines have been operated on a petroleum-derived liquid hydrocarbon fuel boiling in the range of about 300.degree.-700.degree. F. Recently, because of dwindling petroleum reserves, alcohol-hydrocarbon blends have been studied for use as diesel fuel.
The ignition quality of diesel fuels is expressed in terms of "cetene" or "cetane" numbers, which numbers have been developed on a basis very similar to that adopted for measuring the ignition quality of gasoline in terms of octane numbers.
The cetene number refers to a mixture of cetene (1-hexadecene) and alpha-methylnaphthalene, whereas the cetane number refers to a similar mixture of cetane (n-hexadecane) and alpha-methylnaphthalene. The cetene or cetane numbers indicate volumetric percentages of cetene or cetane in the blend. Cetane is normally used because cetene is difficult to purify and is somewhat unstable in storage. Typically, a fuel consumption is assigned a cetane number by matching the ignition performance of a test sample with reference blends of cetane and alpha-methylnaphthalene. The volume percent of cetane in the blend which gives the same ignition quality test performance as the fuel test sample is taken as the cetane number of that fuel.
Cetane number is related to ignition delay after the fuel is injected into the combustion chamber. If ignition delays too long, the amount of fuel in the chamber increases and upon ignition results in a rough running engine and increased smoke. A short ignition delay results in smooth engine operation and decreases smoke.
Petroleum-derived distillate fuels in the diesel boiling range, without any cetane improver, generally have cetane numbers in the range of about 25-60. Cetane numbers in the range of 25-35 are considered low and those in the range of 50-60 are considered top grade diesel fuels. Diesel fuels in the 35-50 mid-range are most common. An object of the invention is to upgrade the low cetane number fuels at least into the mid-range and to increase the cetane value of the mid-range fuels into the upper portion of the mid-range (e.g., 45-50) or even into the premium range above 50.
Through the years, many types of additives have been used to raise the cetane number of diesel fuel. These include peroxides, nitrites, nitrates, nitrosocarbamates, and the like.
More specifically, U.S. Pat. No. 2,934,048 discloses alkyl nitrates such as amyl nitrate and hexylnitrate as cetane improvers.
Other nitrate cetane improvers for diesel fuels, such as dioxane nitrate and 4-morpholine ethanol nitrate, are disclosed in U.S. Pat. Nos. 4,457,763 and 4,421,522, respectively.
The use of alicyclic diazoacetates; di-azo-acetoacetic esters; diazo-acetic acid salts, iso-diazo-acetic esters, and bis diazo-acetic acid esters and salts, as diesel fuel cetane improvers is disclosed in U.S. Pat. No. 2,225,879.
U.S. Pat. No. 2,316,011 discloses organic compounds containing a five-membered heterocyclic ring structure containing both nitrogen and sulfur, such as, 1,2,3,4-thiatriazole; and 1,2,3,-thiadiazole, as cetane improvers for diesel fuels.
U.S. Pat. No. 3,511,623 discloses the use of metal salts of certain azoles as an additive for leaded gasolines. Suitable azoles from which the salts are prepared include aromatic and/or aliphatic hydrocarbon substituted or unsubstituted cyclopenta diazoles, triazoles, and tetrazoles. Such additives are disclosed to improve gasoline engine performance with respect to one or more of octane improvement, rumble, surface ignition characteristics, piston ring wear, and exhaust valve life. As is well known, an octane improver such as tetraethyl lead, can and often does lower the cetane number of a diesel fuel.
U.S. Pat. No. 4,445,907 discloses the use of an amide containing an aminotetrazole group as a corrosion inhibitor for alcohol containing fuels such as gasohols. Likewise, U.S. Pat. No. 4,294,585 discloses the reaction product of an aminotetrazole, formaldehyde, and N-alkylpropylene diamine as a corrosion inhibitor for alcohol containing fuels.
The search has continued for various other compounds which can function as cetane improvers in diesel fuels. The present invention is a result of this search.